Birmingham Final Paper with Fig.pdf

8/12/2019 Birmingham Final Paper with Fig.pdf

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Presented at the 4thEuropean Continuous Casting Conference Oct. 14-16, 2002 in Birmingham, UK

489

ON THE EFFECT OF LIQUID STEEL FLOW PATTERN ON SLAB QUALITY AND

THE NEED FOR DYNAMIC ELECTROMAGNETIC CONTROL IN THE MOLD.

Siebo Kunstreich and Pierre H. Dauby (x)

Danieli Rotelec, Paris Bagnolet, France -(x)

Formerly with LTV Steel, Cleveland, USA.

1. INTRODUCTIONThis paper addresses surface/subsurface quality (slivers, pipe, pinholes, blowholes, inclusion

content) and diversion rate (slab casting abnormality codes) of products cast on high or low-

throughput thick slab machines. Examples from LTV, NKK and TKS pertaining to curved-mold

and vertical/bending casters will be reviewed which make apparent the paramount effect of steel

flow stability and steel flow pattern in the mold on defect occurrence. The first conclusion,

which pertains mostly to slow-speed or wide-slab machines, is that creating or maintaining a

stable double-roll flow is key to eliminating slab defects. The second conclusion, relating mostlyto high-speed casters, is that the intensity of the double-roll flow pattern must not be excessive.

With this in mind, the efficiency of an existing versatile electromagnetic-based technology that

allows accelerating, slowing down and stirring the liquid steel was examined. The analysis

shows good response of the liquid steel to various imposed magnetic fields, presents

corresponding slab/coil quality improvements, and demonstrates the relevance of the proposed

hydrodynamic and metallurgical concepts.

2. THE MOLD FLOW PATTERN CONCEPTSingle Roll and Flow Instabilities as Causes of Steel Defects

It is now well established that steel flow pattern obtained from conventional bifurcated SENs in

slab caster molds is not of any sort (1 - 6). It can be single or double-roll, with high speeds, narrow

widths, low argon flows, deep SENs and steep angles promoting double-roll flow. Little known

is the effect of steel flow pattern on steel quality.

In a paper presented in Madrid in 1998 (1), TKS-Dortmund/Amepa indicated that slabs cast

during a double-roll flow pattern in the mould were identified to be less frequently defective in

comparison to the average production of the caster. The study was referring to fine scale

sliver-type defects on non-stabilized LCAK steel grades. The slabs were very wide (2700 x 220

mm) and typically cast at 1.00 m/min with a 15 downward SEN and 8-10 l/min argon. Thestudy sadly concluded that under the existing casting conditions, the single-roll flow was the

dominant pattern for all SEN designs.

In 2000, the results of a 4-year research project conducted at LTV Steel with PIV and MFC

measurements were published in great detail (Particle Image Velocimetry in the Technology

Center water models and electromagnetic-based steel velocity measurements in the Cleveland-

East curved-mold caster 2, 3). Two conclusions were:

Steel quality is directly related to steel flow stability: unstable flows typically generate fourtimes more defects than stable flows(3).

Pencil pipe rejections on ULC titanium-stabilized grades are directly related to steel flow

pattern(2)

: single-roll flow leads to significantly more defects than double-roll flow.


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New, unpublished data from LTV Steel Indiana Harbor Works offer an opportunity to confirm

and sharpen these conclusions in an extended range of casting speeds and slab widths (Table I).

The numbers were generated using nail board measurements to determine the steel flow pattern

in the mold of the Indiana Harbor No. 2 machine that was revamped to vertical/bending in April

2000 (7, 8). The analysis was prompted by unexpected instances of pencil pipe defects although

all defective slabs had been cast under the same steady casting conditions.EventNo.

SENDepth(mm)

CastingSpeed

(m/min)

ArgonFlow

(l/min)

Slab Width(mm)

Flow PatternSingle ; Double

1. 144 1.16 20.4 1168 S

2. 127 1.65 9.6 1397 D

3. 127 1.65 9.6 1397 D

4. 162 1.65 8.5 1422 D/S

5. 142 1.50 9.6 1422 D

6. 144 1.47 25.5 1422 D

7. 155 1.57 26.9 1422 D

8. 162 1.52 30.3 1422 D9. 144 1.16 31.7 1422 S

10. 165 1.14 19.8 1448 D

11. 155 1.16 31.7 1448 D/S

12. 150 1.27 15.8 1525 D/S

13. 152 1.35 19.8 1525 D

14. 173 1.16 20.4 1525 S/D

15. 157 1.52 19.8 1575 S

16. 173 1.65 28.3 1575 S/D

17. 137 1.19 18.7 1600 S

18. 162 1.04 14.7 1727 D

19. 111 1.14 18.4 1778 S

20. 111 1.14 18.4 1778 S/D21. 122 1.12 10.5 1879 S

22. 122 1.12 10.5 1879 S

23. 109 1.12 10.5 1879 S

24. 122 1.12 10.5 1879 S

25. 137 1.14 17.0 2133 S

26. 137 1.12 17.0 2133 S

27. 137 1.12 17.0 2133 S

28. 137 1.12 17.0 2133 S

29. 122 1.12 17.0 2133 S

Table I Industrial (nail board) data show that steel flow pattern/stability depends

on slab width, argon flow rate, casting speed and SEN immersion depth.

The data clearly confirm the effect of the various casting parameters described above: eachevent, except one, is explainable line by line considering slab width, casting speed, argon

flow and SEN depth.

Interestingly, the comparison of events 2-3 and 21-24 shows that slabs cast at samethroughput, same argon and same SEN depth, had different flow patterns, with the narrower

slabs being double-roll and the wider slabs being single-roll. In more general terms, Fig. 1

shows that slow speeds and wide widths promote single-roll, independently from machine

throughput.

Again, flow pattern was found to affect steel quality: the six pencil-pipe defective slabs that

occurred at Indiana Harbor during the first year of operation (although of much less


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Fig. 1 Slow casting speeds and wide slab widths promote single-roll flow pattern in the mold (nail boardmeasurements; 23 dippings; 1170/2135-mm wide, 260-mm thick slabs; 1.10/1.65 m/min; 110/165-mm

SEN depth; 8/30 l/min argon). LTV data.

Fig. 2 Left-hand side; curved mold machine: single-roll flow pattern increases pencil pipe defects in

frequency and severity (36 defective coils; 1850-mm wide, 230-mm thick slabs; 1.0 m/min; 6.5 l/minargon). Right-hand side; vertical/bending machine: of the six defective slabs, all were single-roll(1880/2135-mm wide, 260-mm thick slabs; 1.17 m/min; 10/17 l/min argon). LTV data.

Fig. 3 Identification of steel flow pattern with nail board measurements. In the double-roll pattern, steel hits

first the narrow faces and creates a standing wave that locally reduces molten slag layer thickness. In thesingle-roll pattern, steel is first lifted to the meniscus and molten slag layer thickness is reduced in thevicinity of the SEN. LTV data.

0

2

4

6

8

10

12

14

Numberofcas

ts

1.1-1.2 1.3-1.5 1.5-1.65

Casting speed [m/min]

Double roll

Single roll

0

1

2

3

4

5

6

7

Numberofcas

ts

1170-1450 1525-1730 1780-1880 2135

Slab width [mm]

Double roll

Single roll

0

20

40

60

80

100

Frequencyofdefectivecoils[%]

1 2 3 4

Pencil pipe severity rating

Double roll Single roll, unstable, unidentified

0

20

40

60

80

100

Frequencyofdefectivecoils[%]

1 2 3 4

Pencil pipe severity rating

Double roll Single roll

0 100 200 300 400 500 600 700

Distance From Narrow Face (mm)

Mold Powder

Steel

Liquid Mold Flux

0

20

40

60

80

100

Thickness

(mm)

0

20

40

60

80

100 DOUBLE ROLL FLOW

0

20

40

60

80

100

0 100 200 300 400 500 600 700


Thickness(mm)

Liquid Mold Flux

0

20

40

60

80

100

0 100 200 300 400 500 600 700


MoldPowder

Liquid Mold Flux

Steel

SINGLE ROLL FLOW


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severity than those observed in the Cleveland curved-mold machine study) were all cast in

single-roll flow conditions (Fig. 2).

Fig. 3 which describes the impact of steel flow pattern on steel meniscus profile, may explain

why double-roll leads to fewer defects than single-roll flows.

Single-roll flow directs steel upwards from the SEN to the meniscus and narrow faces.Typically it will push mold powder, argon bubbles and inclusions to the narrow faces, which

will result in a lack of lubrication around the SEN and a preferential accumulation of

inclusions and bubbles in the edges of the slabs (9, 10). Maintaining a "stable" single-roll flow

is difficult: for given casting speed and slab width, it requires high argon flows and shallow

SEN immersion depths - two conditions that promote slag emulsification (11), pulsating

meniscus velocities and do not reduce meniscus velocities, nor mold level fluctuations.

In contrast, a double-roll flow that maintains the steel flow deep until it hits on the narrow

faces and splits into two loops will more typically weaken the steels momentum and result

in a relatively slower downward steel stream. A potential negative effect of the double flow

pattern is thinning of the molten slag layer near the narrow faces and powder shearing alongthe meniscus if high narrow-face standing waves and excessive steel meniscus velocities are

generated. However, hyperactive double-roll flow situations can easily be mastered by

electromagnetic forces, as will be described in section 4.

3. FLOW INSTABILITIESTransitioning Flows and Transient Casting Operations as Causes of Instabilities

Several instances of transient casting conditions, such as ladle, tundish, SEN tube, SEN depth,

slab width, or casting speed changes are well known from the operating and quality control

personnel. Accordingly, in each steel company, comprehensive lists of slab abnormalities have

been developed and used to redirect/downgrade coded slabs. Generally, the number of slabs that

are diverted after each transient event is determined by cause-and-effect analysis; strictly, it

should be based on the time necessary for the steel flow to stabilize again after a transient

operation (4 minutes after a casting speed change (1), 15 minutes after a tundish change (1), 35

seconds after a slide gate plate movement (12)). Always the number of slabs affected by transient

casting conditions represents several percents of the annual production of a caster and should not

be overlooked.

Little known is the fact that, depending on casting conditions, the steel flow can establish itself

somewhere between single and double-roll. At that point, the steel flow becomes steadilyunstable unsuspected by the operating and quality control personnel! Events 11 thru 16 in Table

I are such examples of unfortunate combinations of speeds, widths, depths and argon flows. The

observation is critical as, very often, the cause/origin of a coil defect cannot be found. It is as if

the defect did occur during "stable" casting conditions. While this should indicate that the defect

is the result of an intrinsic process flaw (like an inadequate steel flow pattern), it is interesting to

observe that mold flow pattern that combines all casting conditions into one single parameter can

correlate well with defects (2, 3).


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Fig. 4 The EMLA mode promotes heattransfer to the meniscus. NKK data.

Fig. 5 The EMLS mode reduces powder-basedinclusions and related pinholes in D&Imaterial (0.18-mm thick beverage canproduct). NKK data.

Fig. 6 The EMRS mode reduces subsurface

blowholes. Test result on low carbon steel withonly traces of soluble aluminium. NKK data.

100

80

60

40

20

0

WITHOUT

EMLSWITH

EMLS

SLIVER-BASEDPINHOLES

ONBEVERAGECANPRODUCTS(Index)

WITHOUT

EMRS

WITH

EMRS

6

4

2

0900 1800 2700

EMLA Current setting [A]

MeniscusTem

perature

Increase

[C]


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4. EMLA, EMLSMaintaining a Stable Double-Roll Flow

It becomes clear from the precedent section that steelmakers need a technology that can maintain

a stable double-roll flow in the mold, independently from slab width, casting speed, SEN depthor argon flow rate. A technology exists that is an effective method to meet that objective (13, 14).

The technology is based on the use of four linear stirrers that are positioned approximately at

mid-height of the mold; on each side of the SEN, behind each broad face. The stirrers induce

horizontally-traveling (AC) magnetic fields. They create forces in the steel that can accelerate or

slow down the steel according to the traveling direction of the fields. In contrast with "passive"

DC electromagnetic systems that can only brake what goes too fast with a braking effect that is

proportional to the actual speed of the liquid steel, these AC fields are "active." They brake with

considerably higher efficiency than DC systems and can also accelerate the liquid steel (actually,

they can put in motion dead zones that do not move). Switching between accelerating and

slowing down functions is possible on-line by appropriate electronic control in function of all

operating parameters as they change during casting.

Benefits of this technology proposed by NKK in 1991 have been described in great detail in the

literature (13-16). Examples include:

Fig. 4: The electromagnetic level accelerating function (EMLA) uses magnetic fieldstraveling from the SEN to the narrow slab faces. It promotes heat transfer to the steel

meniscus, i.e., improves mold powder melting, shortens the length of the solidification hooks

and reduces slivers and frozen meniscus situations.

Fig. 5: The electromagnetic level stabilizer/slowing-down function (EMLS) uses magnetic

fields traveling from the narrow slab faces to the SEN. It decreases mold level fluctuations,

i.e., reduces powder-based inclusions and sticker alarms on high-speed casters.

5. EMRSThe 3

rdFunction of a Flexible Electromagnetic Stirring Technology

The technology of the four linear electromagnetic stirrers mentioned in the precedent section

offers also a third function of stirring (EMRS, an electromagnetic rotative stirring function)

making it a very flexible electromagnetic stirring technology. Indeed, by appropriate selection of

the direction of the four traveling fields (going in the same direction all over one broad face, but

in opposite direction on the other broad face) forces can be generated at the meniscus that will

rotate the steel in a horizontal plane.

Benefits of meniscus rotative stirring, that was initially proposed by Nippon Steel in 1982 (with,

however, a stirrer located high in the mold), concernthe very early stages of steel solidification

at the meniscus (17). Fig. 6 is an example of test results obtained with EMRS at NKK(18) with

stirrers installed at mid-height of the mold. Observe how EMRS, that makes steel velocity and

heat transfer more uniform along the perimeter of the mold and "washes" the solidification front,

significantly reduces subsurface pinholes, blowholes and inclusion content (alumina and

calcium-aluminates, typically). The data refute the opinion that the "washing" effect of the

rotative stirring can only be obtained with a stirrer located high in the mold and confirm that a

same electromagnetic system can be used efficiently for all three stirring functions on a same

caster.


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6. SELECTING THE OPTIMUM EMLA, EMLS, EMRS MODESBreaking Free from the Constraints of a Demanding Product Mix

The selection and the drive of the EMLA, EMLS and EMRS functions on a same caster may

look complex. Actually it is very simple if one remembers the goal of maintaining a stable

double-roll steel flow pattern to minimize defects.

The control concept can easily be explained on a slab-width/casting-speed diagram and includes

three operating modes.

Fig. 7 represents the single- and double-roll flow events of table I in such a slab-width/casting-speed diagram; for simplification, the unstable S/D or D/S events were not

represented. A hyperbola line for a constant casting throughput of 4.6 t/min was added for

reference. The schematic was chosen to show the limits of a "naturally-existing single-roll

domain at 1550 mm slab widths, and of a "naturally-

existing double-roll domain at >1.3 m/min casting speeds and


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case of high-throughput/low-argon casting or the use of EMLA mode to accelerate in case of

low-throughput/high-argon casting. In industrial mode, an F-value that combines slab width,

casting speed, argon flow and SEN design/depth is calculated every 5 seconds, and stirring

intensity/direction is actuated accordingly. Indeed, water modeling and mathematical

simulations conducted at NKK-Fukuyama related F-value to SEN spouting

stream velocity,meniscus flow velocity and the height of the standing wave at the narrow

faces of the mold. Fig. 9 schematizes the break-even line between EMLS and EMLA

operations. For given SEN depth/design and argon flow, this line can be understood as the

iso-throughput line of the ideal operating "window" where no electromagnetic flow control is

required. Moving away from this line, higher throughputs require stronger

Fig. 9 Schematic representation of the three

operating modes. In the double-roll domain,EMLS-EMLA is operated by automatic F-value controller to maintain meniscus flowvelocity within the optimized operationalwindow. In the single-roll domain,permanent EMLA mode is used to convertsingle-roll and unstable flows into optimizeddouble-roll flows. In both domains, EMRS

mode is used for specific metallurgicalpurposes.

500

1000

1500

2000

2500

0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8casting speed [m/min]

slabwidth[mm]

EMLS

EMLA

EMLA

permanent

DOUBLE ROLL

DOMAIN

SINGLE ROLL

DOMAIN

F-value control

EMRS

Fig. 10 The EMLS mode (at 280 A in thisexample) reduces steel meniscus velocitiessignificantly. Excessive braking (at 360 A in thisexample) reverses the double-roll pattern into a

detrimental "returning" single-roll proving theexistence of an optimum window of operation.

The left-hand side diagrams show liquid steeltrajectories in a 2-D mathematical simulation (theSEN is at the far left). Right-hand side diagrams

represent steel meniscus velocities (positivevelocities go from the SEN to the narrow face).

Meniscus velocities without EMLS at 0 Amp coilcurrent are represented in all three diagrams forreference.

Meniscusflowvelocity

(m/min)

Half mold width (m) Half mold width (m)

Coil current = 0 AMoldheihtm

Meniscusflow

velocity

(m/min

)


Coil current = 280 AMoldhei

htm


(m/min)


Coil current = 360 AMoldheihtm


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EMLS settings; lower throughputs require stronger EMLA settings. Wrong settings must be

avoided, as they generate defects. Interestingly, mathematical simulations show that too

strong EMLS settings (i.e., too much braking) induce a third loop in the mold (Fig. 10) which

disturbs the principal double-roll flow, thereby confirming mathematically the existence of

the optimum window represented on Fig. 8.

The second operating mode is permanent EMLA operation and covers the "naturally-existing single-roll domain, specifically pertaining to wide-slab, slow-speed casters. In this

case, the steel flow exiting the SEN needs to be accelerated to transform the single-roll flow

into a steady and optimized double-roll flow. This operating mode also addresses the various

transient casting conditions described in section 3 such as slow-speed head/tail slabs in

sequence casting or the steady unstable situations. In contrast to the general opinion that

EMLS/EMLA technology is only of interest to high-speed high-throughput machines, this

second EMLA mode addresses the majority of the currently existing casters. Industrial

results are not yet available for publication, however mathematical simulations show that the

EMLA function can easily transform a single-roll flow into a double-roll flow (Fig. 11).

The third operating mode is the rotative stirring EMRS mode. Its role is not to maintain thedouble-roll flow pattern in its optimized operating window, but rather to generate a flow

pattern that homogenizes steel temperature gradient along the mold perimeter and washes the

solidification front. For this reason, currently, the EMRS mode is limited to specific steel

grades such as pseudo-rimming, enameling, peritectic, tin plate, and beverage cans.

Coil current = 250 A

Half mold width (m)

Meniscusf

lowvelocity

Half mold width m

Moldh

eight(m)

Coil current = 450 A

Half mold width m


Half mold width m

Moldheight(m)

Coil current = 0 A

Half mold width (m)


Half mold width m

Moldheight(m)

Fig. 11 The EMLA mode (at 450 A in this example)can transform a single-roll flow pattern into a double-roll flow.

Left-hand side diagrams show liquid steel trajectoriesin a 2-D mathematical simulation. Right-hand sidediagrams represent steel meniscus velocities (positive

velocities go from the SEN to the narrow face).Meniscus velocities without EMLA at 0 Amp coilcurrent are represented in all three diagrams forreference.


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7. CONCLUSIONS

The contribution of this paper was meant to be twofold: (1) improve our understanding of the

effect of mold flow pattern on steel quality and, (2) present an electromagnetic-based technology

that allows to stabilize the preferred flow pattern and minimize slab/coil rejections. Specific

findings are:

Steel flow in the mold is affected by slab width, casting speed, argon flow rate, and SENangle/immersion depth.

Steel flow can be single-roll or double-roll, or it can establish itself somewhere betweensingle and double flow making it steadily unstable. Casting conditions changes such as ladle,

slab width, casting speed, argon flow rate, or SEN immersion depth changes are other

conditions that make the flow unstable.

Steel quality is directly correlated to steel flow stability and steel flow pattern, thereforemaintaining a stable double-roll flow is key to optimizing steel product quality.

The need for a versatile electromagnetic technology that can accelerate, slow-down, or rotatethe steel stream in the mold was demonstrated, and the benefits of the three functions were

analyzed.

Acceleration (EMLA) and braking (EMLS) modes can optimize the double-roll flow patternsthat are prevalent on high-speed high-throughput machines. The effect is to minimize slivers,

prevent excessive mold level fluctuations and reduce powder shearing and breakouts alarms.

A new application of EMLA was described that eliminates the negative impact of slow-speedmachines, wide-slabs, and transient-casting conditions. It consists of accelerating single-roll

and unstable/transitioning flow patterns to widen the domain of the preferred double-roll

flow conditions.

Rotative stirring EMRS which is a third mode of stirring available with the same equipment,

was confirmed to be efficient although the stirrers are installed at mid-mold height. Iteliminates the negative constraints of difficult-to-cast grades or demanding applications such

as pseudo-rimming, enameling, peritectic, tin plate, and beverage cans by reducing

subsurface pinholes, blowholes and inclusion content

A qualitative fluid-flow approach (single/double roll) was used in conjunction with aquantitative mathematical approach (F-Factor and 2-D simulation) to propose optimum use

of the three functions.

In its optimum use, the new, versatile technology is a unique tool to free caster operatorsfrom incompatibilities and inherent conflicting constraints between demanding product

mixes (slab width/casting speed/steel quality incoherence), productivity goals, and steel flow

patterns that are naturally unsuitable for defect minimization.

ACKNOWLEDGEMENTS

The authors wish to thank Dr. J. Kubota from the Fukuyama Works Steelmaking Department

and Messrs. T. Kondo and S. Mizuoka from the Steelmaking Group Consulting Department of

NKK Corporation in Japan for their valuable suggestions and discussions and support of this

paper.


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